The sword given to Casia by Solatis at the end of The Call of the Void.
Spoilers from The Call of the Void:
Shadowslayer was created when Solatis removed the dark curse Malaphar put on Casia at the end of A Twist of the Blade. It was drawn from a mass of Solatis’s own energy, combined with Malaphar’s, and threaded together with the Heart of the Sun in its pommel. Its true power is only wieldable by Casia, though Elander is able to use it to great effect as well due to her connection with Casia and with the Heart of the Sun.